A metallic magnetic thin film disk used in a computer disk drive typically comprises a substrate made of aluminum, glass, glass-ceramic or other similar materials plated with a nickel compound such as NiP, NiU, NiNb, NiAl or other nickel compound typically 50% or greater atomic percent Ni (referred to generally as a Ni layer herein), underlayers sputtered onto the plated Ni, one or more magnetic cobalt alloy layers sputtered onto the underlayer, a carbon protective overcoat sputtered onto the magnetic layer(s) and one or more lubricant layer(s) deposited on the carbon.
Before depositing the underlayer, the plated Ni layer is polished to remove surface defects and to lower surface roughness which strongly affects the flying height of a recording head over the disk.
In current and future disks, a smoother polished NiP surface with fewer polish defects is required. At present, the lowest surface roughness Ra obtained using commercially available slurries for Ni plated substrate polishing is about 0.5 nm. ("Ra" is a well-known measure of surface roughness.) However, polishing defects become severe problems as the polished surface roughness is lowered. Two kinds of polish defects, micro-scratches and polish pits, are apt to form on the Ni polished surface. In general, micro-scratches are thought to be caused by large agglomerated particles. Polish pits are formed either by chemical attack or other unknown causes.
One type of commercially available slurry used for polishing NiP plated substrates typically comprises two components: alumina abrasive particles and an acidic etchant. The abrasive particle size ranges from about 0.1 .mu.m to 1 .mu.m. The slurry pH typically ranges from 2 to 6 for various polish process applications. Polishing with these slurries is based on micro-machining, wherein the abrasive has an angular shape and grinds the surface. The acidic etchant helps increase the efficiency of the micro-machining and improves the polished surface finish by chemical etching. Although a smoother polished surface can be obtained by using smaller abrasive particles, it is still not possible to make scratch-free polished surfaces because of the greater hardness of the alumina abrasive compared with the hardness of the surface. Further, these conventional slurries are apt to cause polish pits.
In order to eliminate the above-mentioned problems of conventional slurries, colloidal silica has been considered for polishing NiP plated substrates. Colloidal silica has long been successfully used for polishing various materials, such as silicon, gallium arsenide, indium phosphide and titanium, to form a super-smooth and scratch-free surface finish. In general, colloidal silica is softer, rounder and smaller than abrasives such as alumina, thus allowing colloidal silica to provide the super smooth and scratch free surface. However, because of these properties, the mechanical grinding action of colloidal silicas is less aggressive than other abrasives, thus leading to a typically lower material removal rate.
Various kinds of chemicals are used in colloidal silica slurries for different polishing applications to achieve either an increased material removal rate or better polished surface finishes with fewer polish defects. Alkaline chemicals, for instance, are used as etchants in colloidal silica slurries to reduce surface roughness in semiconductor wafer rough polish processes as described in U.S. Pat. No. 5,571,373 issued Nov. 5, 1996 to Krishna et al., incorporated herein by reference. Persulphate, as described in U.S. Pat. No. 5,575,837 issued Nov. 19, 1996 to Kodama et al., is used as an etchant in a colloidal silica slurry for mirror-finishing metal surfaces.
Unfortunately, several problems are encountered when attempting to use these commercially available colloidal silica slurries to polish Ni plated substrates. For example, existing commercial colloidal silica slurries, either with an alkaline etchant or an acidic etchant, exhibit a very low Ni removal rate. Further, these slurries also cause polish pits, which are caused by chemical attacking, and micro-scratches. There is presently no adequate colloidal silica formulation for polishing NiP.
To make a colloidal silica slurry applicable to NiP plated substrate polishing, a new formulation is desired to increase slurry's NiP removal rate and to decrease polish defects. It is known in the art that adding an oxidizer or changing chemistry can increase the material removal rate or remove polish defects. However, the extent to which one can add an oxidizer or change the slurry chemistry is bounded by colloidal chemistry as described by I. Ali et al. in "Charged Particles in Process Liquids", Semiconductor Intl., pp. 92-95, Apr. 1990. The colloidal suspension may be broken or the aqueous colloidal abrasive can jell due to the pH value change caused by oxidizer addition or other chemistry change. A colloidal silica slurry having an increased material removal rate as described in co-pending U.S. patent application Ser. No. 08/965,099, which application is assigned to the assignee of the present invention and which application is hereby incorporated by reference.
The slurry described in the above-mentioned application showed an increased Ni removal rate and a reduced number of polish defects, and simultaneously provided a good polished surface finish. It would be desirable to provide a slurry that further improves the removal rate while maintaining or improving upon the number of defects and the polished surface finish.